Further effects of chain-length-dependent reactivities on radical polymerization kinetics

In the present paper, we finalize some threads in our investigations into the effects of chain-length-dependent propagation (CLDP) on radical polymn. kinetics, confirming all our previous conclusions. Addnl., and more significantly, we uncover some unexpected and striking effects of chain-length-dependent chain transfer (CLDTr). It is found that the obsd. overall rate coeffs. for propagation and termination (and therefore the rate of polymn.) are not significantly affected by whether or not chain transfer is chain-length dependent. However, this situation is different when considering the mol. wt. distributions of the resulting polymers. In the case of chain-length-independent chain transfer, CLDP results in a considerable narrowing of the distribution at the low mol. wt. side of the distribution in a chain-transfer controlled system. However, the inclusion of both CLDP and CLDTr yields identical results to classical kinetics in these latter two cases, the mol. wt. distribution is governed by the same chain-length-independent chain transfer const., whereas in the case of CLDP only, it is governed by a chain-length-dependent chain transfer const. that decreases with decreasing chain length, thus enhancing the probability of propagation for short radicals. Furthermore, it is shown that the inclusion of a very slow first addn. step has tremendous effects on the obsd. kinetics, increasing the primary radical concn. and thereby the overall termination rate coeff. dramatically. However, including possible penultimate unit effects does not significantly affect the overall picture and can be ignored for the time being. Lastly, we explore the prospects of using mol. wt. distributions to probe the phenomena of CLDP and CLDTr. Again, some interesting insights follow. [on SciFinder (R)